Chapter 8: GSM & CDAMA Systems · PDF fileProvides and manages radio transmission paths between MS and MSC ... Configuration, Selection, Allocation, De ... 1. Broadcast Control Channel

Lecture Notes By: Saroj Dhakal

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Chapter 8: GSM & CDAMA Systems

Global System for Mobile Communication (GSM)

Second Generation (Digital) Cellular System

Operated in 900 MHz band

GSM is also operated in 1800 MHz band and this version of GSM is better known as

DCS 1800

Because of excessive crowding in the above bands, GSM has also started to operate in 2

GHz band

GSM Services

Telephone Services

Bearer Services or Data Services

Packet Switched Protocols

Data rate: 300 bps to 9.6 kbps

Supplementary ISDN Services

Call Diversion

Caller ID

Closed User Groups

SMS

Call Restrictions

etc.

GSM Features

Subscriber Identification Module (SIM)

Memory Device that stores information such as the subscriber’s identification

number, the networks and countries where the subscriber is entitled to service,

privacy keys, and other user- specific information

SIM gives subscriber units their identity

On-the-air privacy

Encryption of bit stream transmitted by GSM transmitter

Secret cryptographic key that changes with time for each user

Unlike analog FM cellular phone systems, virtually impossible to monitor and

listen other’s conversation


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GSM System Architecture

Three major interconnected subsystems:

Base Station Subsystem (BSS) (includes MS)

Network and Switching Subsystem (NSS)

Operation Support Subsystem (OSS)

BSS

Provides and manages radio transmission paths between MS and MSC

Consists of many BSCs connected to MSC

Many BTS connected to each BSC

Handoff between two BTSs under same BSC is conducted by the BSC, MSC informed

Digital Signal Processing

Trans coding and rate adaptation

Channel coding and decoding

Frequency hopping and power control

Radio Resource Control

Configuration, Selection, Allocation, De-allocation, Monitoring, Encryption of

radio channels

NSS

Switching of GSM calls

Connects MSC with other networks: PSTN, ISDN, and other MSC etc.

MSC is the central unit

Contains three different databases:

Home Location Register( HLR)

Visitor Location Register( VLR)

Authentication Center( AuC)


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Functions of MSC Call setup, supervision and release

Call routing

Billing Information Collection

Mobility Management

Network Registration

Location updating

Inter BSS and Inter MSC handovers

Manage connections to BSS, other MSCs and other networks (PSTN, ISDN etc.)

HLR

Contains identities of mobile subscribers

IMSI( International Mobile Subscriber Identity)

SSI( Service Subscription Information)

Service Restrictions (if any)

Supplementary Services

Billing/Accounting Information

Location Information

VLR

Temporarily stores certain information of roaming users

IMSI

Temporary Mobile Subscriber Identity

Location of registration

Once registered in the VLR, certain data from HLR is copied to the VLR for proper

routing of the roaming user’s call

AuC

Strongly Protected Database

Maintains authentication and encryption keys for each subscriber in the HLR and VLR

Equipment Identity Register( EIR)

International Mobile Equipment Identity( IMEI)

May be valid, suspect or fraudulent

OSS

Contains several Operation Maintenance Centers( OMCs)

Monitoring and maintaining of BSS and MSC units

Three functions:-

Maintain telecom hardwares and networks

Manage charging and billing procedures

Manage all mobile equipments in the system


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GSM Radio Subsystem

Originally employed in 900 MHz band but nowadays used in many bands

Uplink ( Reverse Link) : 890-915 MHz ( 25 MHz band)

Downlink ( Forward Link) : 935-960 MHz (25 MHz band)

A 20 MHz guard band : 915-935 MHz

Tx-Rx freq spacing : 45MHz

Tx-Rx Time spacing : 3 TS

Available forward and reverse bands divided into 200 KHz channels :ARFCN ( Absolute

Radio Frequency Channel Numbers)

Each ARFCN time shared by as many as 8 users

A certain time slot of certain ARFCN forms the physical channel

Radio transmission rate( Modulation data rate) : 270.833 kbps

Effective channel transmission rate per user: 270.833/8 = 33.854

Actual user data rate considering overhead( maximum): 24.7 kbps

Bit duration: 3.692 µs

Modulation Scheme : BT=0.3 GMSK

Each TS has an equivalent time allocation of 156.25 bits

Duration of each TS= 576.92 µs

Frame duration= 4.615 ms

Total no. of channel within a 25 MHz band

With no guard band= 125

With guard band of 100 KHz on either side= 124

GSM Channel Types

Two types:

Traffic Channels ( TCH)

Control Channels (CCH)

Traffic Channels carry digitally encoded speech or user data

TCH have similar format in both forward and reverse link

Some CCH may just be defined for forward or reverse link

Control Channels carry signaling and synchronizing information between the BS and the

MS

GSM Traffic Channels (TCHs)

Full rate TCH

User data transmitted in one TS per frame

Half rate TCH

User data transmitted in same TS in alternate frames

TCH data not sent in TS0 in certain ARFCN

Frames of TCH data broken every 13th frame by either Slow associated control channel(

SACCH) or idle frames

Each group of 26 frames forms a Speech Multiframe


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Full Rate TCH

Full-Rate Speech Channel( TCH/FS)

Carries user speech digitized at a data rate of 13 kbps

After channel coding 22.8 kbps

Full Rate Data Channel for 9600 bps( TCH/F9.6)

Carries user data sent at 9600 bps

After error correction coding, user data sent at 22.8 kbps







Half-Rate TCH

Half-Rate Speech Channel( TCH/HS)

Carries user speech digitized at a data rate of 6.5 kbps

After channel coding 11.4 kbps

Half Rate Data Channel for 4800 bps( TCH/H4.8)



Half Rate Data Channel for 2400 bps( TCH/H2.4)



GSM Control Channels (CCH)

Broadcast Channels (BCH)

Operates in forward link of a specific ARFCN

Transmits data only in TS0 of certain frames of broadcast ARFCNs

Provides synchronization for all mobiles within a cell


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Common Control Channels (CCCH)

Forward channels occupy TS0 of every GSM frame that is not used my BCH or

Idle frame in any broadcast ARFCN

Reverse channels occupy TS0 of every frame of broadcast ARFCN

Pages specific MS, assign signaling channels, receive mobile requests for service

Dedicated Control Channel (DCCH)

Bidirectional and may exist in any TS and on any ARFCN except TS0 of

Broadcast ARFCN

Used for signaling services reqd. by users and to supervise data transmission

between MS and BS during a call

Broadcast Channels (BCH)

1. Broadcast Control Channel (BCCH)

Forward Channel

Transmits cell and network identity

Broadcasts info about channels used, channels available etc.

Frames 2-5 in a control multiframe (51 frames)

2. Frequency Correction Channel (FCCH)

Transmitted at TS0 and repeated every 10 frames

Allows subscriber units to synchronize with the frequency of the BTS

3. Synchronization Channel (SCH)

Transmitted at TS0 of a frame immediately following a FCCH frame

Allows each mobile to frame synchronize with the BS

Transmits Frame Number (FN) along with Base Station Identity Code (BSIC)

Issues coarse timing advancements commands to MS so as to synchronize the

received signal with the BS clock

Common Control Channels (CCCH)

1. Paging Channel (PCH)

Provides paging signal to all mobiles and notifies a specific mobile about an

incoming call

Transmits IMSI of target subscriber and requests for ACK in the RACH


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2. Random Access Channel (RACH)

Reverse link channel

Used by subscriber to send ACK for a page or to initiate a call

Uses TS0 of all frames of the broadcast ARFCN

3. Access Grant Channel (AGCH)

Forward Link Channel

Instructs MS to operate in certain physical channel

Used by BS to respond to a RACH signal

Dedicated Control Channel (DCCH)

1. Stand-alone Dedicated Control Channel (SDCCH)

Carries Signaling Data during connection of MS and BS

Ensures connection between MS and BS while MSC verifies the subscriber unit

and allocate resources

2. Slow Associated Control Channel (SACCH)

Maps into the same physical channel as the traffic channel of each user

Used to transmit slow but regularly changing control information such as transmit

power level instructions and timing advance instructions in the forward link

Reverse SACH carries info about received signal quality and strength of

neighboring cells

3. Fast Associate Control Channel (FACCH)

Carries urgent but similar kind of info as SDCCH

Assigned when there is urgent message such as handoff

Gains access by stealing frames from the traffic channel

If the two stealing bits are set in certain TS then it is considered FACCH rather

than TCH


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Frame Structure for GSM

Each of the normal speech frames are grouped into larger structures called multiframes which in

turn are grouped into superframes and hyperframes. One multiframe contains 26 TDMA frames,

and one superframe contains 51 multiframes, or 1326 TDMA frames. A hyperframe contains

2048 superframes, or 2,715,648 TDMA frames.


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CDMA (Code Division Multiple Access)

Frequency and Channel specification:

IS 95 (Interim Standard 95) – 2G standard

Reverse link: 824-849 MHz

Forward link: 869-894 MHz

Each carrier is 1.25 MHz wide

Duplexing: FDD

Multiple Access: CDMA

Maximum user data rate: 9.6 kbps

User data spread to a channel chip rate of 1.2288 Mchip/s

Modulation and Spreading techniques are different in forward and reverse link

CDMA Standards for 3G CDMA 2000 1xRTT

First 3G CDMA standard

Recognized by ITU as IMT-2000 standard in 1999

One times the original cdma One channel bandwidth

Supports user data rate up to 307kbps , throughput rates up to 144kbps

Improvement through rapidly adaptable signaling rate

No additional RF equipment needed to enhance performance, all changes made in

software or in baseband hardware

CDMA2000 1xEV (Evolutionary)

Provided CDMA carriers with option of installing radio channels with data only (

cdma2000 1xEV-DO) or with data and voice (cdma2000 1xEV-DV)

Cdma2000 1xEV-DO dedicates radio channels strictly to data users, and supports greater

than 2.4Mbps of instantaneous high-speed packet throughput

Cdma2000 1xEV-DV supports both voice and data users and can offer usable data rates

up to 144kbps with about twice as many voice channels as IS 95B

MC 3G CDMA

Cdma2000 3xRTT

Uses 3 adjacent 1.25MHz radio channels that are used together to form a 3.75MHz

channel that provides instantaneous packet data throughput speed in excess of 2Mbps per

user

3 non-adjacent channels may also be used together

3G GSM Standards TD-SCDMA

Developed by China as a IMT-2000 3G standard

Combines TDMA /TDD technology with CDMA

Supports up to 384kbps of packet data rate

Radio channels divided into 1.6 MHz bands

Each frame divide into 7 TS which may be provided to a high data rate user or several

slow rate users

Supports Asynchronous Traffic Demand

Uplink Synchronization: Precise tuning of transmission timing of individual terminals

with the BTS


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Smart Antennas, Joint Detection, Terminal Synchronization, Dynamic Channel

Assignment

Low intracellular and intercellular interference, High Spectral Efficiency

Operated in 1900 and 2000 MHz band

Each physical channel is identified by a particular time slot and a particular code on a

particular carrier frequency

WCDMA (UMTS: Universal Mobile Telecommunication System)

3G standard for GSM, PDC and IS-136

Network structure and bit level packaging similar to GSM standards

Fundamental radio air interface technology similar to cdmaOne and cdma2000

Requires minimum spectrum allocation of 5 MHz

Supports packet data rate upto 2.048 Mbps

Fast closed loop power control in both uplink and downlink

BS uses two transmitting antennas for phase and amplitude diversity

Supports high quality data, multimedia, videoconferencing, telemedicine etc.

Requires complete change of RF equipments at the BS.


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Pseudo Noise Code (PN Code)

The PN code is pseudo-random sequences

They are deterministic, periodic sequences which mimic randomness properties.

Randomness or noise-like characteristics include

“1”s and “0”s appear randomly in sequence.

The number of “1” and “0” are (almost) the same in any long segment of sequence

Half of the total no. of run lengths of 0s or 1s are of length 1, then almost 50% of

remaining are of length 2, again 50% of remaining are of length 3 and so on.

Use of PN sequences in CDMA system:

To spread the bandwidth of the signal to larger transmission bandwidth

To distinguish between different user signals by utilizing the same transmission

bandwidth in multiple access scheme.

Generated using LFSRs and modulo-2 adders

N shift registers are used to generate 2N-1 length long PN sequences

A PN sequence and a phase shifted version of itself are nearly orthogonal i.e, no. of

Agreements- no. of Disagreements is nearly equal to zero

Walsh Codes

Perfectly orthogonal to each other.

Different codes have zero cross correlation

IS-95 uses 64 Walsh codes on forward link

Used for call identification

Reserved codes for pilot, paging and sync

IS-95 uses Walsh Codes in the reverse link for modulation

Generated using Hadamard Matrix/Array

PN-Code Generation Mode

(N=5)

X1 X2 X3 X4 Outpu

ttt

X5 C1=0 C2=0 C3=1 C4=0 C5=1

CP


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Spreading

In CDMA, the user data is multiplied i.e, modulo-2 added with a spreading sequence that has

much wider bandwidth than that of the user’s signal. So the resultant signal has a bandwidth in

the range of that of the spreading sequence.

So the information carried by a narrowband signal is spread over a wider bandwidth. This

is called spreading.

Two types of codes are used for spreading the user’s signal:- PN Codes and Walsh Codes

Forward CDMA channel

64 Walsh Coded channels available for use

Consists of Pilot Channel, Synchronization Channel, up to 7 Paging Channels and

remaining and remaining forward traffic channels

Pilot Channel:

Provides phase reference to each MS for coherent demodulation

Transmitted with a signal strength 4-6 dB higher than traffic channels

Allows MS to compare strengths of signals coming from different BS to

determine when to handoff

Always coded with Walsh Code 0

Need to lock onto other logical channels on the same RF Carrier


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Synchronization Channel

Used to broadcast time synchronization messages

Transmits information such as System ID, Network ID, offset from the system

time, paging channel data rate etc.

Operates at a fixed rate of 1200 bps

Always assigned Walsh Code 32

Paging Channel

Used to send control information and paging messages from the base station to

the mobiles

Assigned Walsh codes 1 through 7

Operates in different data rates 9600,4800,2400 or 1200 bps

Control information include no. of paging channels, base station identifier, access

parameter message, channel list message etc

Traffic Channels:

Supports different speech/data rate:- 9600,4800,2400 or 1200 bps (RS1)


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Convolution Encoder

Near Far problem

Near Far Problem

MS near to BS increases the noise floor for the far users

Two Power Control Schemes:-

Open Loop Control

MS adjusts its transmission power automatically by analyzing the pilot

signal

Closed Loop Control

The BS measures the SIR from the MS and then sends command to MS so

that it adjusts its transmission power accordingly


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Reverse CDMA Channel

Consists of Access Channels and Reverse Traffic Channels

Up to 32 Access Channels associated with each Paging Channel

Access Channels used to respond to a page or to initiate a call

Modulation scheme different from that in forward channel

Documents

Chapter 8: GSM & CDAMA Systems · PDF fileProvides and manages radio transmission paths between MS and MSC ... Configuration, Selection, Allocation, De ... 1. Broadcast Control Channel